Cellulose pulping system



'Patented Nov. 3, 1942 CELLULOSE lPULPIN G SYSTEM Lyle Melvin Sheldon,Alton, Ill., assig'nor to The Cellulose Research Corporation, EastAlton, lll., a corporation of Delaware Application May 21, 1937, SerialNo. 143,967

2 Claims.

This invention relates to the production of cellulosic pulp fromresinous woods and in particular to the removal of4 resinous materialstherefrom.

A primary object of the invention is to provide a process for producingpuried cellulosic pulp suitablefor conversion into cellulose esters,particularly acetate, ethers, Viscose and other derivatives fromresinous woods, such as the southern pine species; said purifiedcellulosic pulp being capable of very close duplication with respect toits physical and chemical properties between successive batches.

Another object is the provision of an economical process employingselected wood from `the various species of trees known collectively assouthern pines as a base raw material to. produce purified woodcellulosic pulp capable of acetylation, xanthation and conversion intoother derivatives, which wood has heretofore been considered unsuitablefor the satisfactory production of purified cellulosic pulp of the classdescribed; said process providing means for the removal of impurities,such as resins, which are characteristic of said southern pine species.

A further object is to provide an economical process for producing sultepulp for paper, employing the selected Wood for use from the variousspecies of trees known collectively as southern pines as the base rawmaterial, which woods have been heretofore regarded as unsuitable forthis purpose; said process providing the means for removing impuritiessuch as resins and pitch.

l In the accompanying drawing which illustrates several features of thepresent invention:

Figure 1 is a typical time-temperature impregnation and digestionschedule.

Figure 2 shows the relation of bleaching temperature to viscosityreduction. f

Figure 3 shows the relation of-bleacn1ng time to viscosity reduction andcolor of cellulose. i

The advisability of conning the raw material for the manufacture ofchemical celluloseto one genus or to one species of trees from which thebase wood material is derived has been suggested. In treating evensingle species of wood there is a tremendous distinction'within eachspecies of the factors affecting pulping and purification of the wood,the amount of recoverable alpha cellulose and the economy with which theprocess may be carriedout.

In the process of the present invention there is employed a certaingroup of trees not herepulp suitable for derivative purposes. Theprimary advantage found in this group is the almost complete absence ofheartwood. For example', the group of trees, known collectively assouthern pine (which include the individual species .of Pinuspalustrz'along leaf; Pinus taeda, loblolly; Pinus echinata, short leaf;and Pinus caribead, slash pine), grow in such a manner that if they arecut before they reach the age of about 25 to 35 years, they consist ofsubstantially all sapwood. At .the age. of 29 years, for example, slashpine will produce logs in excess of 8"-10'? in diameter, which obviouslymay be economically chipped by the customary practices.

The southern pine wood species have heretofore proven unsatisfactory forpulping by the sulte processes owing to the serious operatingdifficulties encountered because of their high pitch or resin content'.The pitch and resin-content is frequently as high as 5%-10%'and whenpermitted to solidify before pulping, as by evaporation and prolongedstorage, is quite difficult to remove. In the manufacture of pulp forpaper purposes by the prior practices the resin exhibits a strongtendency to agglomerate, clog the screens and wires of the papermachines and form very undesirable defects in the paper.

I have discovered, however, that by selecting southern pine woodsubstantially free of-heartwood and defective material, including pitchpockets, before the pitch or resin content has had an opportunity tosolidify and preferably before any substantial amount of fungus growthsuch as blue stain develops, a highly uniform cellulosic pulp suitablefor conversion into ce1- lulose derivatives may be economically preparedtoforeknown to be capable of yielding cellulosic according tothe processhereinafter set forth. I have also found that such selected woods may besatisfactorily and economically used for producing strong sulfite paperpulp.

The logs are debarked. washed and chipped by the customary practices.The chips are preferably of the order of M1" in length, but the standard3A" to V8 chips may be employed, if desired.

The suitably divided wood is impregnated with the selected delignifyingreagent. Normally, it is preferredin connection with the presentinvention to use a sulte delignifying solution, such as ammonium,calcium or sodium bisuliite. The chips'are impregnated and thoroughlysaturated with the delignifying liquor at a temperature of C. or less,while at the same time substantially their entire air content isremoved. At a temperature of the order of 100 C. there is but slighttendency for deligniflcation to take place, hence the saturation of thechips is `accomplished before the delignication of the wood proceeds. Bythis provision a far more uniform treatment of the woody material isinsured.

Impregnation of the wood with the digesting liquor may be accomplished,for example, by boiling the chips in a freely vented tank or digesterfilled with the treating solution and then allowing the surroundingliquor to flow into the interstices of the wood either by increasing thepressure on the liquor or by cooling the mass slightly below 100 C. Ineither case, the steam within the interstices of the wood is condensed,thus forming a partial vacuum which draws the surrounding liquor intoall the capillaries and interstices.

Removal of the air from the subdivided wood and impregnation with thedigesting liquor may also be accomplished by a preliminary boiling ofthe wood with water to remove the air and then adding the delignifyingliquid to the chips and providing a suiilcient time to permit diffusionof the delignifying chemical through the water saturated chips.

'Regardless of the particular digestion procedure, the character of theraw material employed together with the advantageous presaturation ofthe chips with the delignifying solution makes it possible to carry outthe digestion under such mild and lenient conditions that substantiallyno chemical degradation of the cellulosic material in the wood isencountered an a high yield results.

The mild and lenient digestion results in the production of pulpYwherein the cellulosic fibers have a gradually increased porosity andpermeability over their original condition and which are relieved of thegreater portion of their ligneous impurities. The digestion moreoverreduces the pitch content of the pulp to less than the order of about1.5%.`

At the completion of the digestion, the treating solution is drained andthe reaction products washed from the pulp. Preferably with hot waterand with air excluded. This treatment not only improves the color of thepulp but also aids in removing a part of the residual pitch and otherhot water soluble impurities from the fibers, such for instance ashydrolyzed hemicelluloses and mineral constituents composed mostlyA ofcalcium, magnesium, iron, manganese, sodium, potassium and siliconsalts. The pitch content of the digested pulp may be reduced evenfurther by adding 1/4% or less caustic soda, based on the dry weight ofthe pulp, to the hot water used in washing.

Upon the completion of the thorough washing of the crude pulp, it isscreened to remove knots and other fragment material and then completelydeflbered by any suitable mechanical means; the debered pulp may bescreened, if necessary, to remove any neps or bundles of bers which mayhave survived the deflbering process.

Regardless of the ultimate use of the cellulosic pulp, whetherforcellulose acetate, viscose products or for bleached paper pulp, therst step in the purication is a uniform chlorination of the deilberedpulp. The` chlorination may be accomplished by introducing a knownamount of chlorine gas into a water slurry of the pulp under highlyefllcient agitation. -For economy and to insure complete deligniilcationof the pulp, the gas is added to the pulp slurry in as short a time aspossible consistent with uniform distribution., An amount of chlorine inpounds for less.

each 100 pounds of pulp (oven dry basis), equivalent to 14%18% of theVbleach value of the pulp is employed. The control of the distributionand amount of chlorine 4results in a maximum purification of the pulpwith an almost complete absence of chemical degradation.

Following the chlorination, the pulp is treated with either hot or-coldalkaline solutions to remove the reaction products. The result of thechlorination and causticizing treatments is to reduce the bleach valueof the pulp to between the order of 1.0% and-5%, depending upon thekindof alkaline treatment given, and to reduce the pitch content to lessthan the order of 0.3%.

Regardless of the bleachability of the washed pulp after thechlorination and causticizing treatment within the range of 1% to 5%, itis bleached in a dilute hypochlorite solution containing 1% to 6%bleaching powder based on the pulp. A sufiicient amount of alkali isadded tc the bleach solution to bring its pH value to approximately 8.0or above. Under these treating conditions the residual oxidizableimpurities remaining in the pulp are reacted upon'in the early stagesoi' the bleaching, for example, in from l to 11/2 hours or less.

The treatment is continued for an additional time, usually from 2 to 3hours, to reduce the cuprammonium viscosity of the cellulosic pulp tothe desired value. By reference to Figure 3 it will be observed that thecuprammonium viscosity of the pulp is rapidly reduced during the earlystages of the bleaching treatment. This is followed by a period in whichthe rate of reduction is relatively small. For example, it will be seenthat the viscosity is reduced from 42 centipoises to about 39.0centipoises .in 1 hour, but in the succeeding 3 hours the value isreduced only an additional 2.2 centipoises. Thus by clontinuing thehypochlorite treatment until the relatively level portion of theviscosity curve is reached, the treatment may be readily terminated atthe vdesired viscosity level, since the rate of reduction is relativelysmall and the desired time to give the required value may be calculatedwith precision.

'I'he desired reduction in the cuprammonium viscosity of the pulp mayalso be effected by varying the temperature of the hypochlorite bleachtreatment and holding the time, concentration, and consistency of thesolution constant.

Upon the completion of the bleaching treatment the pulp is thoroughlywashed with water until free of available chlorine, preferably untilneutral.

At the end of the bleaching treatment the cellulosic pulp will have analpha cellulose content of about %94%, depending upon the time,temperature and concentration of alkali used in the treatment afterchlorination. The resin content will be reduced to the order of 0.3% orThe material is very satisfactory for use in preparing'viscous productsand is characterized by its nearly invariable physical and chemicalproperties between successive batches. It is also very satisfactory forpaper purposes since it possesses unusually high strength properties, ahigh white color, and the resin content is in such condition and of suchlow value as to cause none of the usual clogging of the screens orlulose content.

The washed pulp may be given a second hypochlorite bleach employing lessthan 1% bleachsuitable port above the water level. This forces the watersurrounding the chips out of the di' gester. After all of the water hasbeen drained from the chips in this manner, the water outlet ing powderbased on the dry weight of the pulp.

The time, temperature and consistency are controlled substantially asdescribed for the rst hypochlorite treatment. The pulp is again washeduntil free of available chlorine, prefer- 'ably lmtil neutral.

The. cellulosic pulp at this stage of its purification is ofexceptionally high quality for use in the preparation of viscoseproducts. The alpha cellulose content is approximately 95%-97%, its sodasolubility less than 4% and the resin content is reduced to the order of0.25% or less. It may also be used to advantage in the preparation ofacetate where exceptionally low color and haze in solvent solutions arenot essential.

When the cellulosic pulp is intended for conversion into celluloseacetate of high quality. the pulp obtained by the treatments above ismercerized in a strong vsodium hydroxide solution.

Upon the completion of the mercerization it is important to wash thecaustic reagent from the cellulose as rapidly as possible and in theabsence of atmospheric oxygen. lWith suitable equipement, this may beand is desirably accomplished in 30 minutes or less.

Cellulose acetate of lower cost may be obtained from the use ofthepuried pulp of the present invention since the base raw wood material isnot only cheaper than other available supplies of raw cellulosicmaterial, but its ready response to chemical treatment permits importanteconomy in thel process of its conversion to acetylatible quality.

In vorder to more particluarly describe the present invention and mannerof' attaining uniformity in the reactions, the following example setsforth a typical embodiment thereof:

Example A conditions. It is important that the chipsbe mosphere at thetop of the digester is closed and the steam supply to the bottom portsis cut off.' A suitable outlet in the bottom of the digester fordraining is then opened and steam is admitted at the top of the digesterthrough a at the bottom and the steam inlet at the top of the digesterare both closed. This leaves the digester and the voids around thechips, the capillaries and interstices within the chips completelyfilled with steam at approximately 100 C.

The ammonium bisulte liquor previously preheated to a temperature of theorder of C. is then pumped into the digester in suiiicient quantity toattain a ratio of liquor tooven dry wood of the order of 6:1. When theliquor comes in contact with the chips the steam with which they aresaturated is forced to condense. This creates a partial vacuum whichacts to draw the treating solution at a temperature just under Steam isnow admitted into the digester and the temperature raised at a uniformrate to the order of 1219-123" C. over a period of 3 hours, as shown byreference to curve AB', Figure 1. The temperature is held within thisrange for approximately 3 hours (curve B'iC', Figure l). During thisperiod the sulfonation of the noncellulosic components of the wood takesplace. Owing to the completeness with which the air has been evacuatedfrom the digester and the chips before the temperature is raised toabove 100 C., the pressure within the digester during the sulfonationcycle (curve B'C', Figure ,1) will be due solely to the heated treatingsolution and should not exceed 85 ypounds per square inch. Gas reliefsto prevent the pressure exceeding the customary limit in theconventional digester equipment are consequently usually unnecessary.

Thus, the usual variability in the concentration of the treating liquorduring the sulfonation period, occasioned by the loss of sulfur dioxidegas by venting, is avoided and a treatment of the h woody material witha liquor of nearly invariable composition between successive batches isinsured.'

At the end of the 3 hour sulfonatlon period, the temperature is raisedto the order of C. at a uniform rate over a period of l hour (curve C'D,Figure 1) and maintained closely at this level for from 2-4 hours (curveD'E', Figure l), depending upon the bleachability and viscosity desiredin the digested pulp. The eiect upon the bleachaoility and viscosity byvarying the period of treatment at a temperature of the order -of 135 C.will be described later in this example.

Hydrolysis of the suli'onated, non-cellulosic material takes placesatisfactorily during the treatment at temperatures4 in excess of 130C., and in the present example the digestion is terminated at the end of2 hours treatment at 135 C. Gas relief to control the pressure in thedigester below the permissible limit of the equipment may be 'practicedduring the hydrolyzing cycle. Care is exercised that the venting iscarried out in such a manner as to avoid reducing the temperatureappreciably below the preferred'level of the order of 135 C., since therate of hydrolysis of the sulfonated, ligneous material is aected byvariations in temperature. Short gas reliefs 'of Thus, by first lessthan 30 seconds each will accomplish the desired reduction in pressurewithout substantially lowering' the temperature, except in the -finalstages of digestion where gas relief is practiced to effect as completerecovery of sulfur dioxide as possible.

Upon the completion of the digestion schedule, the pulp is -blown fromthe digester in the Vcustomary manner into a blow pit and washed withwater, preferably as near 100 C. as possible until it is neutral tolitmus. .The washed l pulp is screened through a 0.012"0.015" screen andpassed over a riiile box for further removal of heavy foreign material,such as knots, particles of bark, dirt and the like.

By the foregoing digestion schedule, from 50-52 pounds or more of ovendry pulp are obtained from each 100 pounds of oven dry slash pine woodused. An actual analysis of a typical screened pulp showed thefollowing:

It will be noted that the retention of the native alpha cellulose is44-46 pounds of each 100 pounds of oven dry slash pine wood used. Theresin content is reduced to the order of 1% and is in a condition whichpermits its ready removal to an inconsequential residue devoid ofobjectionable agglomerating tendency in the iirst step of purificationto be presently described.

The digestion schedule just described is designed to produce a highyield of raw pulp of relatively high viscosity and bleachability. Thesame schedule and liquor composition applied to other coniferous woodssuch as spruce, or to decidous woods such as black gum, yellow birch andthe like will result in raw pulps whose -viscosities and bleachabilitiesmay vary to some extent 'from the abovevalues.

Consequently, it is usually necessary to determine theprecise-adjustment of the time-'temperature schedule to give a. pulp ofthe desired bleach and viscosity characteristics by carrying out one ormore experimental digestions on the quality of wood to be used incontinuous operation. The adjustments in the digesting practice forcontrolling the bleachability and viscosity of the raw pulp are Thesevariations will also` occur with wood of the same species obtain fromdifferent geographical areas and of vary-` ing growth conditions.

conveniently made by varying the time or the temperature, in some casesboth. To obtain a maximum yield of raw pulp together with highcuprammonium viscosity, the average cooking temperature should be heldas low as possible and the time of treatment prolonged by experiment togive a pulp that can be readily and completely deilbered.

High cuprammonium viscosity in the raw pulp is quite necessary where itis intended that the purified cellulose shall have a viscosity in excessof the order of 25 centipoises. There are certain types of celluloseacetate, however, for which it is necessary to provide a puriedcellulose having a relatively low cuprammonium viscosity, for example,below the order of 10 centipoises. For

this purpose it is more convenient to produce a 7s 'the time being heldconstant.

' Ash raw pulp with a cuprammonium viscosity less than the order of25-30 centipoises. To accomplish this the digestion schedule,represented by curve DE, Figure 1, is carried out at a somewhathigher-temperature, for example, 140-145 C., This treatment appreciablylowers the cuprammonium viscosity and also effects a considerablysmaller proportional reduction in the bleach value. The purpose ofproviding a relatively low raw pulp viscosity as the starting materialto produce puried cellulose of less than 10 centipoise viscosity will beexplained in the description which follows of the hypochlorite bleachtreatment.

After the washed, digested pulp has been thoroughly deiibered, theconsistency is adjusted to 3 %l0% and the pulp slurry pumped to a gastight chlorination tank equipped with an eilicient agitation device. Thepulp is nowtreated with an amount of chlorine gas equal (in pounds perpounds of oven dry pulp) to 16% of the bleachability value of the pulpas determined by the permanganate number method. This amount will justsatisfy the demand of the immediately reactive lignin and should becontrolled within 3% or less. Larger amounts of chlorine areuneconomical and tend to cause chemical degradation of the cellulose,whil smaller amounts result in incomplete chlorination.

The required amount of chlorine gas is incorporated in the pulp slurryat a uniform rate of yiow in between 20 and 30 minutes. The chlorinewill be substantially exhausted in an additional 5 to. 15 minutes butthe treatment is allowed to continue for a total of 45 minutes (afterall of the chlorine has been added) in order to allow the acids formedby the chlorination treatment to act upon the ash content of the pulp.The

chlorine should be added in a manner that will insure a uniformdistribution of gas with the. unchlorinated fibers; otherwise anincreased loss of chlorine may occur due to reaction with materialalready chlorinated and result in an incomplete chlorination of part ofthe fibers. It

has been-found desirable to effect the incorporationof'the requiredamount of chlorine gas into the pulp slurry in as short time as possibleconsistent with such uniform distribution as will avoid the`beforementioned undesirable localized action.

At the end of the 45 minute chlorination treatment, a suillcient amountof sodium hydroxide is added to the chlorinated pulp to .bring theconcentration of the solution to about 1% and the I 'thoroughly withwater, preferably until neutral.

A typical sample of the chlorinated and causticized pulp has thefollowing analysis:

Per cent Alpha cellulose 94.9 Soda vsolubility 6.5 Cuprammoniumviscosity (ctps.) 54.0 Bleachability 1.1 Pentosans 5.4 Resins 0.3

Ithas been' found that the viscosity of the finall purified cellulosewill be approximately '75%-85% of its value at the end of thehypochlorite bleach treatment. In general, the higher the viscosity ofthe purified end product, the greater will be the spread between thevsicosity value at the end of the hypochlorite treatment and such nalviscosity. For example, if a viscosity of 30 cen-- tipoises for the nalpuried cellulose is desired, the pulp viscosity should be reduced in thehypochlorite treatment to approximately 35 centipoises. If an endproduct of 18 centipoises is desired, the cuprammonium viscosity of thecellulose should be reduced by'means of the hypochlorite treatment toapproximately 20 centipoises.

In carrying out the bleaching step of this example, the required volumeof hypochlorite so.- lution to give. 2.5 pounds of bleaching powder(calculated on the basis of 35% available chlorine) for each 100 poundsof pulp is added to the pulp slurry which has previously been adjustedto a consistency of 2.7%. -This addition of reagent will give aconcentration of bleaching powder in the treating solution at the startof the reaction of0.'06'l%. The hypochlorite is added in the form of asolution containing 60-'70 grams of bleaching powder per liter andsaturated with lime. The bleaching reaction is allowed to proceed for 4hours at a temperature of 25 C. which resultsl in a consumption ofapproximately 50% of the active reagent present. Control and closeduplication of the pH conditions during the' bleaching step ofsuccessive batches is essential for reproduction of the rate and extentof viscosity reduction. The preferred practice is to have the pH at thestart of the reaction at about 9.4 and allow it to decrease uniformly toa value about 8.7 at the end of the treatment. The lower limit of pH forsatisfacviscosity. Variations such as the rate and degree of agitation,pH during bleaching, size of batch, direct or indirect heating, rate ofheating, presence of metallic contaminants etc. are factors in thecontrol of viscosity during bleaching. I have found a control viscositydetermination to be a practical method for obtaining a substantiallyconstant cuprammonium viscosity of successive .batches of pulp after thecompletion of the hypochlorite bleach. A curve showing the change inviscosity with time'of bleaching should first be determined on a numberof successive lots from the same wood supply going through the process.This is desirable in order that the cumulative effects may be knownofthe natural variations in the raw material and the .minor unavoidablevariations in the processing steps arising from the limitations ofaccuracy in measuring the weight and consistency of pulp, weight ofreagents and the like. When these curves areA viscosity ofthe pulp uponthe termination of 'the bleaching. Reference to Figure 3 shows thatalthough in the last two hours oi the bleaching the change in viscositywith time -is small, proper adjustments in time can be made to readilyoverments in time may be made with no deteriment to the other pulpproperties for the oxidizable material in the cellulose has beensubstantially tory bleaching 'of pulps lintended for use in derivativesis of the order of 8.0.

The bleaching treatment just described affects primarily thecuprammonium viscosity, the

color of the cellulose and the amount of oxidirar ble material present.g the course of the bleaching step, the cuprammonium viscosity will bereduced to about 43 centipoises, the color of the cellulose improvedfrom a reflectance value of about 82.4% to a value of about 93.4% andthe bleachability reduced to a value of about 0.4% f

or less. v

. If a greater reduction in the viscosity of the pulp is desired thanillustrated by the present example. this may be eiected by employinghigher bleaching temperatures or by adding agreater percentage ofbleaching powder. However, either practice tends to undesirably impairthe properties of the pulp and furthermore are lessv reacted with duringthe iirst hour of the treatment -andthere is practically no change inthe alpha cellulose and soda soluble contents even though the bleachingtreatment should be prolonged 2 hours or more beyondv the 4 hourspreferably chosen and used in this example.

A typical analysis of the cellulose after bleach- Per cent Alphacellulose '94.6 Soda solubility..-` .L '1.9 Cuprammonium viscosity('ctpsJ. 43.0

Bleachability 0.35 Pentcsans 5.2 Resins A0.3

gld (based onoven dry.wood).. `4245, or moreA y The'cellulose at thisstage of its puriiication may be used toprepare viscose products;however, to obtain a higher quality of cellulose further purincationtreatments may be given. It is also of excellent quality for use in themanufacture of paper. It should be noted that the resin content of thepulp has been reducedto a negligible valueV and furthermore, it has beenfound that the condition in which the residue exists is such that it hasnotendency to-agglomerate and cause the objectionable clogging of thewires and felts, typical of pulps produced fromresinous woods by theprior practices when such pulps are used for the manufacture of paper.l

This is considered an important feature of the invention since it makespossible the use of the abundant supplies of low cost southern pines forpaper purposes.

Aftervwashing until free of available chlorine, the consistency of thepulp slurry may be adjusted to about 4%-12% and the pulp treated with a'1% caustic soda solution for 11/2 to 3 "hours at about 100 C. underconditions which allow a minimum amount of atmospheric oxygen to come incontact with the pulp. Following this treatment the pulp is drained freeof excess solution and washed thoroughly with water until the pH hasbeen reduced to between 7.0 and 8.0

Yield (based on oven dry wood) 38-40, or more This cellulose may b eacetylated according to the practice .disclosed in Letters Patent No.2,187,710 of Lionel Goff et al., granted January 16, 1940; or, it may beused as a high quality raw material for preparing vviscose products.

A further variation of the purification practice consists in-omittingthe calcium hypochloritebleaching treatment and proceeding directly tothe 7% sodium hydroxide boiling treatment as soon as the chlorinatedcompounds have been washed from the pulp.. In this case, after thecaustic boiling treatment, the pulp is 'thoroughly washed in purifiedwater, preferably until neutral, and given a 0.5% to 1.0% calciumhypochlorite bleach treatment in which the concentration of activereagent and/or temperature is adjusted to give the desired reduction inviscosity. 'I'he analysis ofthe pulp will be similar` tothat produced bythe first described practice and is a particularly desirable quality forxanthation purposes, as well as for conversion into useful acetate andother esters".

Where the highest qualities of acetate or other esters are desired,particularly those acetates for use in film and sheeting, the `pulp isnext merthe caustic soda still retained by the pulp washed out undercarefully controlled conditions with puried water, preferably untilneutral. 'Ihe ated even more economically and satisfactorily by themethod set forth in copending application of Lyle Sheldon et al., S. N.70,372, filed March 23, 1936, which has become abandoned. It should benoted that the resin content has been reduced to the low value of 0.1%.

A more thorough understanding of the advantages inherent in the use ofsapwoods with substantially complete exclusion of heartwood andcompression wood as a raw material for the production of chemicalcellulose can possibly be derived by considering the morphologicaldifferences between sapwoods and heartwoods.

Sapwood may be considered as that portion of the xylem nearest to thebark which contains living cells and which serves for the conduction,support, and storage of food; while the heartwood functions mainly asmechanical support, after all active growing ceases in the sapwood andit vserves only as mechanical tissue, it becomes heartwood which iscomposed of dead wood cells and contains a greater amount of resin,gums, etc. than is present in sapwood.

Sapwood may be distinguished from heartwood i in most species of wood bythe difference in color;

usually the latter is of a much darker color. 'I'he infiltration of thecell walls with such substances as oils, resins, and coloring matter andthe plugging or-lling of the lamina of the cells with gums, resins, andwaxes are largely responsible for the change in color. southern pines,particularly long leaf and slash pines, is impregnated with resin whichsolidifles in the -lumen of the tracheids, resin ducts, and parenchymacells. The resin of the sapwood of these species is in the liquid stateand is confined for the most part in the resin canals. In

and around injuries and knots, the'wood is often saturated with resin,even in the sapwood portion.

In the transforation of sapwood into heartwood a number of importantchanges occur; all

living cells losetheir protoplasts; the cell sap is withdrawn .andcommonly the water content.

of the cell walls is greatly reduced; food material present in theliving cells is removed; and

the partly lignifled walls or parenchyma cells properties'of thecellulose'are, especially affected by the procedure followed in removingthe alkali from the cellulose at the completion of the mercerizingtreatment. The time employed to' remove the caustic soda from the pulpshould be no more than the order of 30 minutes andis preferably l5minutes or less.

The'mercerization (and classification, if employed) completes thechemical purification of the cellulose, a typical sample of which willthen analyze as follows:

I Per cent Yield (based on oven dry wood) 36738, or more The abovehighly purified cellulose can now be dried and acetylated by thewellknown methcds ordinarily practiced. Itis, however, preferablymaintained in a wet condition u. :l acetylmay become more stronglylignifled. There are formed within, or brought into the changing cells,certain substances new to the tissuesl such as oils, gums, resins,tannin compounds, and various achromatic and coloring substances, theachromatic substances being such as the colorless salts of metalsmagnesium, silicon, calcium,

sodium and potassium, as wellas some colorless organic compounds. Theso-called it .membranes, in the case of coniferous spec es, which act asvalves permitting the flow of ,fluids to and fromthe interior of theindividual wood or cell fibers becomes fixed in a closed position.

By virtue of having used only selected sapwood as the starting materialin the present process and the careful control of the conditions underwhich each step of the digestion of the wood and the purification of thepulp are carried out, an

end cellulosic pulp product is produced having any desired degree ofpurity and distinguished by the nearly invariable chemical and physicalproperties between successive batches.

The control of the chemical treatment in each purication step almostcompletely avoids chemical degradation of the cellulosic fibers andeffects a gradual increase in the purity of the product as measured bythe alpha cellulose and soda soluble contents. At .the same time,pitch.. resins and The heartwood -of the other objectionable impuritiesare almost completely eliminated with the result that the cellulosicpulp product of the present invention possesses a homogeneity combinedwith high purity and permeability which renders it a nearly idealproduct for conversion to cellulose derivatives.

The alkaline treatment of the chlorinated pulp is quite eiective inremoving the residual pitch and resin compounds which have heretoforebeen a troublesome problem in the use of highly resinous wood pulps in-paper making. Furthermore, the process of the present invention notonly removes these objectionable resinous materials, but producessubstantially resin-free pulp of high whiteness with a yieldconsiderably higher than that ordinarily obtained by the prior practicesin producing similar pulp from the nonresinous wood species.

In view of the above, it will be seen that the several objects of theinvention are achieved and other advantageous results obtained.

As many changes could be made in carrying out the above compositions andprocesses without departing from the scope of the invention, it isintended that all matter contained in the above description shall beinterpreted as illustrative and not in a limiting sense.

Since no published methods have been found which clearly dene certainmeasurements used herein, the following denitions of such treatments aregiven.

Soda. soluble material is dened as that portion of a cellulosic sampledissolved when it is sub- .jected to the action of l7.14% sodiumhydroxide solution at the boiling point of water for 3 hours by arefinement of the method of Griiiin, Techl nical Methods of Analysis,492 (1927 ed.), described in Letters Patent cf Sheldon et al. No.2,185,776 granted January 2, 1940, 'page 2, column 2. v

Alpha cellulose is defined as that portion of a sample of cellulosematerial not dissolved by 17.5% sodium hydroxide solution at 20 C.,determined by a refinement of the method described by H. F. Lewis inTechnical Association Papers,

Series XVII, #1, 436 (1934) described in Letters Patent of Sheldon etal.No. 2,185,776 granted January 2, 1940, page 2, column 1.

Bleachability of cellulose, as used herein, is a measure of thematerials oxidizable by potassium permanganate in the presence of anacid under specific conditions, and is expressed in terms of standardbleaching powder containing 35% available chlorine. method, includingthe conversion table for expressing the permanganate number in terms ofper cent .bleaching powde` of 35% available chlorine, was published byT. A. P. P. I., Series XVII, #1, 146 (1934) Permanganate Number of Pulpby R. N. Wiles.

Cuprammomum viscosity, as' used herein,

means the viscosity number or value obtained according to the followingmethod: The cuprammonium solution was prepared by the action of air onelectrolytic copper in the presence of strong ammonia water. The copperconcentration of the solutions employed for viscosity determinations was30, i 2 g. per liter and the ammoniaI content Was 165 g., i 2 g. perliter. The concentraticn of cellulose employed was 0.6 g. (oven drybasis) per 100 cc. of cuprammonium solution. The cellulose sample forthis determination was dried at C. to 4% moisture content. Afterweighing out 0.6 g. (oven dry basis), the sample A detailed descriptionof the was moistened, squeezed to a uniform weight of tion in anatmosphere of hydrogen 'from which oxygen has been completely removed.The viscosity measurements were made at 25 C. with a modied Ostwaldpipet, constructed according to the specifications of the AmericanChemical Society Committee on the Viscosity of Cellulose (Journal ofIndustrial & -Engineering Chemistry, I, #49; 1929). The time of flow inseconds was converted to centipoises on the basis of the calibration ofthe pipet with oils of known viscosity in centipoises obtained from theUnited States Bureau of Standards.

Colo@` and haze- The measurements referred to herein of the color andhaze of the acetic acid and acetone solutions of the acetate were madeby comparisons with standards of known color and also turbidityexpressed in parts per million. The standards are those used for wateranalysis recommended by the American Public Health Association andconsist of platinum cobalt (for color determinations) and fullers earth(for turbidity comparisons). With the materials recommended by thevAmerican Public Health Association, a series of standard solutionsranging from 10 p. p. m. to 200 p. p. m. inclusive at' intervals of 10p. p. m.'were prepared for both color and haze. cc. of each of thesesolutions was hervmetically sealed in an 8-ounce, wide-mouth.glass-stoppered bottle having an internal diamhour after the dope hasbeen/killed? by the addition of the dilute acetic acid. At/-the time ofmeasurement, the dope must bei'labsolutely free 1 from air bubbles andat a temperature of 25 C., and the quantity of dope under comparison inthe bottle must not be less than 100 cc. It has been found that colorand haze ratings by this method can be made to an accuracy ofapproximately 10 p. p. m.

Color of cellulosa-The color of the cellulose is measured by the amountof light reflected from a pad of cellulose. This is determined by usinga photo-electric cell and is expressed as per cent of the range fromblack to the white of a magnesium carbonate block.

I claim:

1. The process of producing cellulose pulp from highly resinous woodsuch as, Southern' pine, comprising providing fresh Southern pine indesired subdivisions having their resin content in a liquid state in thenatural resin solvent, impregnating the subdivisions with a sulphite di-.vesting liquor by 'heating the subdivisions to a temperature at whichsteam is formed Within the subdivisions. and condensing the steam whilethe subdivisions are submerged in a liquid solvent of the sulphiteliquor, then progressively raising the temperature to the order of121123 C., maintaining the temperature in the range below C. for atleast'three hours with substantially no gas relief, thereafter raisingthe temperature above 130 C. and continuing the digestion for at leasttwo hours at the higher temperature to 2,300,738 simultaneously reducethe resin content to less treatment, then treating the pulp in a.caustic solution to remove the reacted products of the chlorinaton andsimultaneously reducing the resin content to 0.3% or less, and thenbleaching said pulp to desired whiteness with hypochlorite.

`2. The process as set forth in claim 1 in which the hypochlorite bleachis followed by a treatment of the pulp with a 7% caustic soda solutionat elevated temperature for from one to three hours further to reducethe soda soluble content of the pulp to less than 3%.

` LYLE MELVIN SHELDON.

